Electrical power source

ABSTRACT

A power source capable of providing a constant DC output when supplied with one of a range of AC inputs. Converter means providing a number of outputs of differing voltages each proportional to the input voltage. A number of parallel circuit lines each connected to one of those outputs. Each parallel circuit line including a rectifier, a maximum voltage regulator and a non-return current device. The outputs from those lines being connected to a common output to give the constant DC voltage supply. Each voltage regulator has a set regulating voltage which is one of a discrete range of voltage steps close to the required substantially constant DC voltage, the lowest of the steps being no greater than the voltage reached for the line connected to the highest output from the converter means when the input supply is at the low end of the range of AC voltages, the next of the steps being greater than the lowest and being reached for the line connected to the next highest output from the converter means, and, if there are more than two parallel circuit lines, the next step being greater and being reached for the line connected to the next highest output from the converter means and so on, so as to cover the range of possible input AC voltages.

This invention relates to electrical power sources.

Many pieces of domestic electrical equipment need to be capable ofworking with different mains supply voltages, the most common which arethe 110 volts supply existing, for example, in the United States ofAmerica and the 220 volts supply existing, for example, in Europe. Insome countries the standard voltage is slightly different from thesestandard voltages whilst in other countries the supply itself may have arather erratic and variable value.

To meet the differences between a 110 volts and a 220 volts supply,equipment such as electric shavers may use a simple resistor which canbe manually switched into and out of circuit to dissipate the excess ofthe larger voltage. Alternatively a transformer can be used where alarge power supply is concerned but it is still necessary to adjust thetap position manually to match the supply and in many cases adjustmentfor the relatively small changes from the two standard voltages is notprovided for. Also no account is taken of erratic changes in voltage.

It is therefore an object of this invention to provide a simple powersource which can accept a wide range of AC power input voltages yetwhich will automatically provide a relatively constant DC output.

According to the invention there is provided an electrical power sourcecapable of accepting an input within a range of AC input voltages andproviding a substantially constant DC output supply, comprisingconverter means for providing from the AC input supply a number ofoutputs of differing voltages each proportional to the input voltage,each of these outputs being connected respectively to one of a number ofparallel circuit lines each of which includs a rectifier, a maximumvoltage regulator and non-return current device, the outputs of theselines being connected to a common output intended to provide thesubstantially constant DC voltage supply, each voltage regulator havinga set regulating voltage which is one of a discrete range of voltagesteps close to the required substantially constant DC voltage, thelowest of the steps being no greater than the voltage reached for theline connected to the highest output from the converter means when theinput supply is at the low end of the range of AC voltages, the next ofthe steps being greater than the lowest and being reached for the lineconnected to the next highest output from the converter means, and, ifthere are more than two parallel circuit lines, the next step beinggreater and being reached for the line connected to the next highestoutput from the converter means and so on, so as to cover the range ofpossible input AC voltages.

Such a power supply can be very simple to make and in addition canoccupy a relatively small space. It can, therefore, fit easily withinmany domestic electrical appliances. Also the supply automaticallyaccepts any AC voltage within the designed range of AC input voltage andgives a substantially constant DC output. Further the power sourceaccepts automatically any erratic and fluctuating changes in the inputvoltage substantially within affecting the output DC voltage.

Typically there will be four parallel circuit lines to cover an input ACvoltage range from 80 to 260 volts.

In order to improve the stability of the output DC voltage the commonoutput can be fed through another maximum voltage regulator having a setoutput voltage which is lower than any of the set values of those in theparallel circuit lines. In this way even though there will be smallvoltage changes as a result of current being drawn from one or other ofthe parallel circuit lines depending on the input voltage, the finaloutput voltage can be very stable. In some cases, however, this furtherstep may not be necessary because the relatively small differences inoutput from the various parallel circuit lines may insignificant to thepiece of equipment receiving the DC output.

An important advantage of the invention is that the power source has lowhigh-frequency noise and so a low risk of radio interference.

A power source according to the invention will now be described, by wayof example, with reference to the accompanying drawings, in which:

FIG. 1 is a circuit diagram of one form of power source according to theinvention;

FIG. 1A is a detail showing a modified embodiment;

FIG. 1B is a detail showing another modified embodiment;

FIG. 1C is a detail showing a further modified embodiment; and

FIG. 2 is a graph illustrating the operation of a power source accordingto the invention.

The power source 10 shown in FIG. 1 includes a transformer 12 which hasa primary 14 and secondary 16. The primary 14 receives an input ACvoltage V across the terminals 18 and 20. The secondary 16 has a numberof tappings providing outputs at points 22, 24, 26 and 28.

Each of the points 22 to 28 is connected to one of the parallel lines A,B, C & D. Each line includes a rectifier 30, a voltage regulator 32 anda diode 34 to prevent return current flow. The lines A to D all join ata common point 36 which provides a substantially constant DV voltageoutput v at the output 38.

The rectifiers 30 can be a half-wave rectifier composed of a diode 40and capacitor 42 as shown in FIG. 1 or can be a full-wave rectifier asshown in FIG. 1A.

Equally the transformer 14 could be replaced by a resistor with tappingsto provide voltages corresponding to those at the points 22 to 28.

Each voltage regulator 32 comprises a transistor 50 whose base is heldat a particular voltage by a resistor 52 and a common Zenner diode 53,and if appropriate one or more diodes 54. In this way depending upon thevoltage supplied to its base, each transistor 50 will give an output DCvoltage at its emitter which reaches a maximum predeterminal level whenthe voltage on its collector reaches a certain value and will not exceedthis even when the voltage applied to the collector exceeds that saidcertain value.

It will be seen that the voltage which is applied to the base of therespective transistors 50 varies from one of the lines A to D toanother. Indeed the arrangement is such that the voltage at the base inline A is lower than that at the base in line B, the voltage at the basein line B is lower than that at the base in line C and that the voltageat the base in line C is lower than that at the base in line D.

The AC voltage V which is supplied to the primary 14 of the transformer12 is within a predetermined ranges and as an example it is convenientto design the power supply so that it will accept AC voltages within therange of 80 to 260 volts. This will then embrace the full range ofcommonly existing mains supply voltages and in particular the American110 voltage and European 220 voltage.

Reference will now be made to FIG. 2 to illustrate the operation of thepower source 10. The voltage above ground at the points 22 to 28relative to the input voltage V is designated by the lines A' to D',respectively.

In turn the maximum predetermined voltage level passed by thetransistors 50 in the lines A to D at their collectors is given by thelines a to d, respectively, whilst the voltage v at the output is givenby the thick line 60.

In the example shown the maximum voltage levels a to d are say, 5.1,5.2, 5.3 and 5.4 volts, so that the maximum variation in output voltagev is from 5.1 to 5.4 volts depending upon the value of the input voltageV in the range 80 to 260 volts.

The lines A' to D' show the voltages resulting at the points 22 to 28 asthe input AC voltage varies. Thus, at an instantaneous input voltage of80 of 100 volts AC, the line A' will have reached a figure of 5.1 voltsDC which will then be fixed at that substantially constant value by thevoltage regulator 32 in the line A. If on the other hand the inputvoltage is in the range of about 100 to 140 volts then now it will beseen that the line B' will have reached the figure of 5.2 volts at whichit will in turn be fixed by the regulator 32 in the line B. Then any DCcurrent drawn from the voltage v at the output 38 will come from theline B, whilst the diode 34 in the line A prevents any back current flowthrough that line.

In similar fashion between about 140 and 210 volts input, the line Creaches a steady maximum voltage of 5.3 volts and output current atpoint 38 will be drawn from line C, and above about 210 volts input theline D reaches the steady maximum voltage of 5.4 volts and the outputcurrent is drawn from the line D.

In many cases this variation in v from 5.1 to 5.4 volts depending uponthe voltage V is sufficiently constant for the piece of equipment withwhich the power source is associated. However, if a more constant supplyis required, then a further voltage regulating step may be required.This is achieved by passing the output from the point 34 to anothervoltage regulator 64, which can be identical with the regulators 32.However, the set voltage for this regulator must be lower than that forthe lowest of the lines A to D, namely line A. In the example shown inFIG. 2, if the set maximum voltage of the regulator 64 is 5.0 volts,then the small variation from 5.1 to 5.4 can be avoided and theresulting final DC outputs voltage u at an output 66 will besubstantially constant at 5.0 volts.

The precise construction for the voltage regulators 32 and 64 is notcritical. For example as shown in FIG. 1B each regulator may have itsown zenner diode 68a having the appropriate threshold voltage or asshown in FIG. 1C each regulator may include a Darlington pairarrangement.

Although the diodes 34 are shown, they will often not be necessary sincethe transistors 50 will act as diodes to prevent return current flowsalong the lines A to D.

What we claim is:
 1. An electrical power source capable of accepting aninput within a range of AC input voltages and providing a substantiallyconstant DC output supply, comprising converter means for providing fromthe AC input supply a number of outputs of differing voltages eachproportional to the input voltage, each of these outputs being connectedrespectively to one of a number of parallel circuit lines each of whichincludes a rectifier, a maximum voltage regulator and non-return currentdevice, the outputs of these lines being connected to a common outputintended to provide the substantially constant DC voltage supply, eachvoltage regulator having a set regulating voltage which is one of adiscrete range of voltage steps close to the required substantiallyconstant DC voltage, the lowest of the steps being no greater than thevoltage reached for the line connected to the highest output from theconverter means when the input supply is at the low end of the range ofAC voltages, the next of the steps being greater than the lowest andbeing reached for the line connected to the next highest output from theconverter means, and, if there are more than two parallel circuit lines,the next step being greater and being reached for the line connected tothe next highest output from the converter means and so on, so as tocover the whole range of possible input AC voltages.
 2. An electricalpower source as claimed in claim 1 in which the converter meanscomprises a transformer with a number of output taps corresponding tothe number of parallel circuit lines.
 3. An electrical power source asclaimed in claim 1 in which there are four parallel circuit lines tocover a range of input voltages of from 80 to 260 volts.
 4. Anelectrical power source as claimed in claim 1 in which a Zener diodeacts as the maximum voltage regulator.
 5. An electrical power source asclaimed in claim 1 in which the DC voltage from common output of theparallel circuit lines is fed through another maximum voltage regulatorhaving a set output voltage which is lower than the lowest of the setvalues of the parallel circuit lines.